Method of recovering the solvent for acrylonitrile polymer from an aqueous solution thereof



14, I and n are positive amnion F IRECOVERING THE SOLVENT FOR ACRYLONITRILE POLYll/IER FROM AN AQUE- o s SOLUTION THEREOF Masamoto Watanabe, Kyoto, and Tadao Yamada, Hideo Sakai, and Masao Takahashi, Shiga-ken, Japan, assignors to Toyo Rayon Kabushiki Kaisha, Tokyo, Japan, a

aqueous solution thereof which comprises adding to said. aqueous solvent solution of an inorganic salt represented by the following general formula:

Ki mMm.

wherein M is an element of either group IV or Vof the periodic table havingan atomic number of at least about a positive integer.-

Heretofore, in the wet spinning of acrylonitrile fibers the solvents for acrylonitrile polymer, for example, vdirnethylsulfoxide and dimethylformamide, were recovered in the form of an aqueous solution thereof. In recovering the solvent from such aqueousvsolvent solutions, due i to the relatively great affinity of dimethylsulfoxide and dimethylformamide to water, and dimethylfor'mamide, the method heretofore, practi ced was generally that in which the solvent was separated. by, distilling the aqueous:

solution. On the otherhand, when dimethylformamide or ethylene carbonate wasused, potassium carbonate (K CO has been employedto aid in the salting and sept Q the selyeets lutiqat,

IN However, these convent'onal methods are not neces sarily satisfactory in that e ther the results of the separation are not completely satisfactory orthe separatingoperations are troublesome. As a result of our researches for a method of recovery in which the solvent advantageously and readily could be separated and recovered from such an aqueous solution of the solvent for acrylonitrile polymer on a commercial scale and moreover in which preferably the spinning operation could be carried out while effecting the continuous recovery of the solvent, we found that by dissolving an inorganic salt corresponding to the above formula in the aqueous solution of the solvent for acrylonitrile polymer, the treated aqueous solvent solution subsequently separated upon standing into two layers, one layer consisting of the polymer solvent 4 I and the other layer being an aqueous solution of the inorganic salt treating agent, thereby readily accomplishing the recovery of the solvent with high purity. In addition, we found that by lowering the temperature of the aqueous. inorganic salt solution, the organic salt could be readily separated from the solution and that by re- 7 cycling and reusing the so obtained inorganic salt the spinning operation could be carried out while continuously recovering the solvent.

Accordingly, the primary object of the present invention is to provide a method of advantageously and readily recovering an acrylonitrile polymer solvent of high purity from an aqueous solution thereof.

7 Another object of the invention is to provide a method of recovering acrylonitrile polymer solvent from aqueous solutions thereof which are obtained in continuous polyacrylonitrile fiber spinning operations.

an aqueous solvent solution integers, and m is either zero'or" i d States Patent .2' 7 Other objects and advantages of this invention will become apparent from the following description thereof. 3

While the inorganic salts to be used in this invention formula may be any that are represented by the following general K n Mo wherein M is an element of group IV or V of the periodic table having an atomic number ofat least about'14, l and n are positive integers,'and m is either zero or a positive integer; from the standpoint ofease of recovery as well 'as preparation and handling of the inorganic salt, K PO KH AsO4, and the like are preferred. Specific examples of other suitable potassium salts include K TiO K TiO In adding the inorganic'salts used in the invention to} the aqueous solution of the solvent for acrylonitrile polymer, they suitably may be added inthe form, of their respective aqueous solutions. However because it is more possible to utilize any of the normally practiced stirring means. When theresulting treated liquid is allowedtostand after stirring and dissolution, it separates into two layers, the upper layer being the solvent layer and the lower, the aqueous solution layer of the inorganic salt.

1 Normally, when polyacrylonitrile; fibers are wet-spun,

the solvent used for th e acrylonitrile polymer in the spining process includes such as dimethyl s ul'foxide, dimethylformamide, ethylene carbonate, etc:, all ofyvhich' are. readily recoverable by the method of this invention. I

In addition, 'when the polyacrylonitrile solution obtained 'by the solution polymerization method ,is spun,

needless to say,;the unpolymerized' acrylonitrile that;is

mixed; in the aqueous solution can also beseparated and:

recovered together with the polymer solvent, by the meth od of this invention. r

The actualquantity of the above-described inorganic salts added to the aqueous solvent solutions depends upon the solubility of the salts, the quantity of water contained in the aqueous solution, and the particular temperature of the aqueous solution. The amount of inorganic salts required for separation operations increases as the con tent of the solvent in the aqueous solution becomes small. In order to carry out as completely as possible the separation into the solvent layer and aqueous inorganic salt solution layer, it is preferred that the addition of the inorganic salts be of a quantity that is sufiicient to saturate the water contained in. the aqueous solvent solution at the operational temperature. Although the operation is carried out at a temperature generally within the range of l0-80 C., depending upon the concentration of the solution, a temperature outside of this range is, of course, usable. In order that the separation and recovery of the solvent is achieved as completely as. possible, it is necessary that stirring is accomplished as thoroughly as possible.

After separation of the resulting solvent and aqueous inorganic salt solution layers, since a small amount of water and inorganic salts still is present in the solvent '--Patnted May 12, 1964.

d of the present invention has the advantage in providing that a given amount of the inorganic salt treating agent and polymer solvent can be employed in continuous polyacrylonitrile fiber spinning operations, the inorganic salt beingjemployed to carry out a continuous separation and recovery of the solvent. 1

The present invention, when compared with the conventional methods, for example, the separating operation involving only distillation, is also of very great advantage from the standpoints of its heat economy, scale of the equipment and ease of operation.

For a clearer understanding of the invention, the following specific examples illustrating the mode of practicing the same aregiven. It is to be understood, however, that these examples are intended to be merely illustrative of and not limitative to the invention, and that many changes and modifications are possible without departing from thescope and spirit of the invention.

Example 1 Potassium dihydrogen arsenate (KH AsO was added to 400 parts of a aqueous dimethylformamide solution, at 20 C. and atmospheric pressure with thorough stirring, in an amount suflicient to saturate the water contained in said aqueous solution, and the resulting'mixture was allowed to stand, whereupon separation thereof into two layers was effected. When the upper layer was separated, 120 parts of a 65% aqueous dimethylformamide solution were obtained;

Example 2 wherein M is an element having an atomic number of at least about 14 selected from the group consisting of elements of group IV and elements of group V of the periodic table, I and n are positive integers, and m is selected from the group consisting of zero and positive integers, to effect the separation of said aqueous solvent solution into two layers consisting of a layer of' said solvent and a layer of an aqueous solution of said potassium salt and separating said two layers.

2. The method according to claim 1 wherein said aqueous solvent solution is obtained in an acrylonitrile polymer spinning process. 7 I, v

3. The method according to claim 1' wherein said aqueous solvent solution is at a temperature in the range of from about 10 to about 80 C. V

4. The method according to claim 1 wherein subsequent to the separation of said two layers, said aqueous solution of potassium salt is cooled to effect the precipitation of said potassium salt, said precipitated potassium salt is recovered therefrom and thereafter is recycled to contact additional aqueous solvent solution.

5. The method according to claim 1 wherein M is an element of group V of the periodic table having an atomic number in the range of from 15 to 51.

6. The method according to claim 5 wherein said potassium salt is KH AsO 7. The method according to claim 6 wherein said solvent is dimethylformamide.

8. The method according to claim 6 wherein said sol- V vent is dimethylsulfoxide.

Having thus set forth the nature of the invention, what we claim is:

1. A method for separating and recovering a solvent' selected from the group consisting of dimethylformamide, dimethylsulfoxide and ethylene carbonate from an aqueous solution of said solvent which comprises adding to 9. The method according to claim 5 wherein said potassium salt is K PO .v

10. The method according to claim 9 wherein said solvent is dimethylformamide.

11. The method according to claim 9' wherein said solvent is dirnethylsulfoxide.

References Cited in the file of this patent UNITED STATES PATENTS Sakurai et al. Sept. 1, 1959 

1. A METHOD FOR SEPARATING AND RECOVERING A SOLVENT SELECTED FROM THE GROUP CONSISTING OF DIMETHYLFORMAMIDE, DIMETHYLSULFOXIDE AND ETHYLENE CARBONATE FROM AN AQUEOUS SOLUTION OF SAID SOLVENT WHICH COMPRISES ADDING TO SAID AQUEOUS SOLUTION AN INORGANIC POTASSIUM SALT REPRESENTED BY THE FORMULA 